There are reasons to do that, women and men react differently to long-term microgravity.
I found this study about the differences (but it's from 2014, please tell me if you find a more recent one) The Impact of Sex and Gender on Adaptation to Space: Executive Summary.
TL;DR female astronauts have, according to the study:
- a higher risk of cancer (a 45-year-old man has a 344-day limit in space versus a 187-day limit for a 45-year-old woman)
- more orthostatic intolerance
- more UTIs (which makes sense as women on earth are also more likely to have UTIs)
- less vision impairment compared to male astronauts (no clinically significant cases of VIIP syndrome)
- less hearing problems (men show a more rapid decline in the left ear and in general as well, but this is the case on Earth too)
Keep in mind that this data is not the best because only around 20% of people that had been on the ISS at the time of the study were women and because male astronauts are more likely to come from a military background.
I personally think this means we should send an equal amount of both sexes (maybe more women as it would help get more data on the differences). Sending all-men or all-women missions sadly won't fully solve the problem as sexual assault isn't always between men and women, that said, it is less common so it's not a bad idea.
Anyways, more investigation is required. If you find a newer/better study please tell me (or if something I wrote is inaccurate), I don't want to spread misinformation